遇到proto编译问问，看看proto语法，记录一下
- protobuf3 语法指南
http://colobu.com/2017/03/16/Protobuf3-language-guide/
https://developers.google.com/protocol-buffers/docs/proto3?hl=zh-cn#generating
- protobuf2 语法指南
http://colobu.com/2015/01/07/Protobuf-language-guide/
https://developers.google.com/protocol-buffers/docs/proto